I did play on core difficulty (“intended PF experience” or something), but as far as I can tell nothing affects the enemy HP, just how much damage you take (maybe there’s something in the custom modifiers). I guess even with a weak character you can get through on a lower difficulty, but it’ll take ages.
However, even if I used proper builds, I still used my very limited knowledge of these games and systems. If you know what you’re doing, you can probably use better spells or abilities to kill things faster.
Check out isthereanydeal.com, for a history of sales.
It looks like it was on sale a few days ago, and will probably be on sale again next week for the Fall Sale and Winter Sale after